Asymmetric membranes comprising a porous support and a thin dense palladium layer are useful for the separation of hydrogen from other gases such as carbon dioxide and other small molecules such as hydrocarbons and other hydrides. A cost-effective production of palladium-based membranes with electroless plating of porous supports requires the presence of palladium seeds on the porous support. For growing thin dense Pd membranes substantially free of defects, the seeds should be homogeneously distributed over the support in a sufficient amount.
Asymmetric membranes comprising a porous support and a thin dense silver layer are useful for the separation of oxygen from other small molecules. (see e.g. http://www.anorg.chem.uu.nl/PDF/Bergwerff_silver%20literature.pdf)
Collins and Way (Ind. Eng. Chem. Res. 1993, 32, 3006-13) use multiple pre-treatments of the support with tin chloride followed by acidic palladium chloride immersion before repeated electroless plating with palladium-amine complex. Li et al. (Catalysis Today, 56, 2000, 45-51) similarly use tin chloride pre-treatment followed by acidic palladium amine before enhanced electroless plating driven by osmosis. Paglieri et al. (Ind. Eng. Chem. Res. 1999, 38, 1925-1936) proposed an improved seeding procedure of dipping the inside of the support into a palladium acetate solution, with chloroform as the solvent, followed by drying and calcining. The resulting membrane had a thickness of ˜20 μm and the H2/N2 selectivity was not higher than 50. Zhao et al. (Catalysis Today, 56, 2000, 89-96) used activation by slip-casting with a Pd-modified boehmite sol, followed by drying and calcining. The use of a boehmite sol is also described in CN 1164436 and U.S. 2008-176060. A pre-treatment with a silica sol is described in KR 2001-045207 and KR 2001-018853. Hou et al. (WO2005/065806) use a boehmite sol as a pore filler before seeding following the tin chloride procedure. After calcining, γ-alumina is formed in the pores restoring the porous structure of the support. Harold et al. (U.S. 2008/0176060) use two γ-alumina layers to sandwich the palladium seeds (layer) applied by electroless plating acting as nuclei for growing palladium in the pores of the top γ-alumina layer also by electroless plating.
These prior art methods lead to insufficient performance of the membranes thus obtained in terms of hydrogen flux, H2/N2 selectivity and stability. The tin chloride pre-treatment results in the presence of tin contamination, which affects both the stability of the plating bath and the temperature stability of the palladium membrane. Even when the pre-treatment does not contain a tin salt pre-treatment, like the improved procedure by Paglieri that uses palladium acetate only, the membrane is thick and disappointingly low selectivities of 50 are obtained. The use of chloroform as a solvent during fabrication is a further disadvantage of their procedure. The use of boehmite sols and the like may result in blocking pores and thus reduce separation performance. It may also reduce the maximum application temperature because of limited thermal stability. Also, the prior art methods do not always allow very thin palladium layers to be produced.
It is therefore an object of the invention to provide a process for producing thin membranes based on palladium, which leads to improved performance of the palladium-based separation layer, and which allows the production of separation membranes having very thin (<5 μm) layers of palladium.